// Copyright (c) 2012 The Chromium Authors. All rights reserved.
// Use of this source code is governed by a BSD-style license that can be
// found in the LICENSE file.

#ifndef BASE_SEQUENCED_TASK_RUNNER_HELPERS_H_
#define BASE_SEQUENCED_TASK_RUNNER_HELPERS_H_

#include "base/debug/alias.h"
#include "base/macros.h"

// TODO(akalin): Investigate whether it's possible to just have
// SequencedTaskRunner use these helpers (instead of MessageLoop).
// Then we can just move these to sequenced_task_runner.h.

namespace tracked_objects {
class Location;
}

namespace base {

namespace subtle {
    template <class T, class R>
    class DeleteHelperInternal;
    template <class T, class R>
    class ReleaseHelperInternal;
}

// Template helpers which use function indirection to erase T from the
// function signature while still remembering it so we can call the
// correct destructor/release function.
//
// We use this trick so we don't need to include bind.h in a header
// file like sequenced_task_runner.h. We also wrap the helpers in a
// templated class to make it easier for users of DeleteSoon to
// declare the helper as a friend.
template <class T>
class DeleteHelper {
private:
    template <class T2, class R>
    friend class subtle::DeleteHelperInternal;

    static void DoDelete(const void* object)
    {
        delete reinterpret_cast<const T*>(object);
    }

    DISALLOW_COPY_AND_ASSIGN(DeleteHelper);
};

template <class T>
class ReleaseHelper {
private:
    template <class T2, class R>
    friend class subtle::ReleaseHelperInternal;

    static void DoRelease(const void* object)
    {
        reinterpret_cast<const T*>(object)->Release();
    }

    DISALLOW_COPY_AND_ASSIGN(ReleaseHelper);
};

namespace subtle {

    // An internal SequencedTaskRunner-like class helper for DeleteHelper
    // and ReleaseHelper.  We don't want to expose the Do*() functions
    // directly directly since the void* argument makes it possible to
    // pass/ an object of the wrong type to delete.  Instead, we force
    // callers to go through these internal helpers for type
    // safety. SequencedTaskRunner-like classes which expose DeleteSoon or
    // ReleaseSoon methods should friend the appropriate helper and
    // implement a corresponding *Internal method with the following
    // signature:
    //
    // bool(const tracked_objects::Location&,
    //      void(*function)(const void*),
    //      void* object)
    //
    // An implementation of this function should simply create a
    // base::Closure from (function, object) and return the result of
    // posting the task.
    template <class T, class ReturnType>
    class DeleteHelperInternal {
    public:
        template <class SequencedTaskRunnerType>
        static ReturnType DeleteViaSequencedTaskRunner(
            SequencedTaskRunnerType* sequenced_task_runner,
            const tracked_objects::Location& from_here,
            const T* object)
        {
            return sequenced_task_runner->DeleteSoonInternal(
                from_here, &DeleteHelper<T>::DoDelete, object);
        }

    private:
        DISALLOW_COPY_AND_ASSIGN(DeleteHelperInternal);
    };

    template <class T, class ReturnType>
    class ReleaseHelperInternal {
    public:
        template <class SequencedTaskRunnerType>
        static ReturnType ReleaseViaSequencedTaskRunner(
            SequencedTaskRunnerType* sequenced_task_runner,
            const tracked_objects::Location& from_here,
            const T* object)
        {
            return sequenced_task_runner->ReleaseSoonInternal(
                from_here, &ReleaseHelper<T>::DoRelease, object);
        }

    private:
        DISALLOW_COPY_AND_ASSIGN(ReleaseHelperInternal);
    };

} // namespace subtle

} // namespace base

#endif // BASE_SEQUENCED_TASK_RUNNER_HELPERS_H_
